Implementations of the claimed invention generally may relate to wireless communication, and in particular to schemes for avoiding recurring interference due to certain transmissions or retransmissions.
Modern wireless data communication systems such as WiMAX, WiMAX-II, 3GPP LTE are designed to combat wireless channel distortions. Orthogonal Frequency Division Multiple Access (OFDMA) is chosen over other techniques due to its excellent capability of dealing with a multipath channel, together with the multi-user diversity. With OFDMA, other methods of combating errors such as Forward Error Correction (FEC) and Hybrid Automatic Repeat Request (HARQ) are typically used.
The scheduling of HARQ retransmission may be either synchronous or asynchronous. For synchronous HARQ retransmission, the retransmissions will occur in predetermined locations (time and frequency) relative to the first transmission. Thus, once the first transmission is scheduled in the downlink (DL) (at slot 110) and/or uplink (UL) (at slot 120), the resource(s) 110/120 for future retransmissions are reserved, as shown in FIG. 1A. No additional scheduling/resource allocation is required for each retransmission 110 and/or 120.
In asynchronous HARQ, by way of contrast, each retransmission will be rescheduled explicitly. It has the flexibility of choosing the best time and frequency allocation. The price to pay for asynchronous HARQ relative to synchronous HARQ is the overhead associated with indicating the scheduling information for each retransmission.
Voice over internet protocol (VoIP) has been identified as one of the key applications for WiMAX, WiMAX-II and 3GPP LTE systems. Such systems may have to support a lot of VoIP users with a constant data rate and small packet size traffic. In such cases, the overhead of indicating resource allocation, the so-called media access protocol (MAP) overhead, may be very significant with VoIP traffic.
One effective way to reduce the MAP overhead is called persistent scheduling. Persistent scheduling allocates resource(s) to a particular user or a group of users in a recurring pattern over a long period of time. For example, as shown in FIG. 1B, resources associated with a particular exchange or conversation may be periodically scheduled at a known position 130 in both DL transmission frames and UL transmission frames. Persistent scheduling works well with VoIP traffic, because the VoIP traffic pattern is very predictable. Thus, in persistent scheduling, the allocation of resources only needs to be made once over a relatively long period of time.
Although they are based on different mechanisms (e.g., synchronous HARQ transmissions and persistent scheduling), there are conceptual and visual similarities between FIGS. 1A and 1B, namely the predetermined, periodic (re)transmissions. Because both may include regular, structured resource allocation, both mechanisms may be susceptible to repeated interference with another similarly structured transmission.
For the mobile stations at a cell's edge, the dominant reason for packet loss is typically interference from nearby stations. There are several ways to mitigate interference, such as power control or beamforming. Power control denotes limiting the transmission power to the nearby stations to avoid interference to other stations in adjacent cell. Beamforming denotes focusing the transmission power to the desirable station only.
When synchronous HARQ or persistent scheduling is employed, data transmission may repeat at certain, fixed locations. In a multi-cell deployment, interference may occur in a repeating pattern. For example, when the neighboring cell has the same synchronous HARQ retransmission latency or persistent scheduling period, then if a transmission in the neighboring cell interferes with a first transmission, it will continue interfering with all the following transmission/retransmissions. If such interference is strong, it may cause significant performance degradation for the whole duration of HARQ retransmission or persistent scheduling. Two exemplary interference scenarios follow.
A downlink interference example is shown in FIG. 2A, with one main base station (BS) 210 and another interfering base station 220. STA11 230 and STA12 are the two example mobile stations (STAs) associated with (e.g., within its hexagonal cell area) the main station 210, while STA21 240 and STA22 are the two stations associated with the interfering station. STA11 230 and STA21 240 are located at the cell edge and are close to each other. This example may assume both BSs 210/220 have implemented beamforming. When STA11 230 and STA21 240 are scheduled at the same time to receive packet from each base station, beamforming is not efficient and a packet loss is very likely. A packet collision may occur, because STA11 230 and STA12 240 are close to each other. Both stations 230 and 240 may request a retransmission. In a synchronous HARQ retransmission scheme, the retransmissions from BSs 210 and 220 will likely collide again. If a persistent scheduling scenario were implemented in the system of FIG. 2A, a similar collision around stations 230/240 would happen for each packet over the persistent allocation period.
An uplink interference example is shown in FIG. 2B, again with one main base station 210 and another interfering base station 220. STA11 230 and STA12 are the two example mobile stations associated with the main station 210, while STA21 240 and STA22 are the two stations associated with the interfering station 220. STA11 230 and STA21 240 are at the cell edge and are close to each other. In this example it may be assumed that the STA11 230 to main BS 210 link happens at the same time as the STA21 240 to interfering BS 220 link. With uplink omni-directional transmission, STA21 240 (shown transmitting to both BSs 210/220) may cause strong interference to main BS 210. Thus, packet loss is likely for both the main BS 210 and interfering BS 220. Both base stations may request a uplink retransmission. Both stations 210 and 220 may request a retransmission. In the synchronous HARQ retransmission regime, the retransmission may collide again. In a persistent scheduling case, a similar collision may happen for each packet over the persistent allocation period.
Hence, synchronous HARQ retransmissions, periodic scheduling, and/or any other type of periodic transmission, may experience periodic interference.